In electric power stations, a boiler brings heat to the fluid of a closed loop circuit, this heated fluid then flowing into a steam generator in order to yield its heat to the water which is converted into steam, this steam then being sent to the turbines of the power plant.
In certain nuclear power stations, particularly of the fast breeder type, it is known to use liquid sodium as a fluid serving for the transport of heat from the boiler to the generator. In this case, the steam generator is constituted by a heat exchanger the primary circuit of which contains liquid sodium and the secondary circuit of which contains water converted to steam.
In such steam generators, very special precautions are taken to avoid any contact between the liquid sodium of the primary circuit and the water of the secondary circuit. In fact, it is known that the mixing of sodium and water at high temperature causes very violent chemical reactions, with a release of gas and sudden increase in the pressure existing in the liquid sodium. These accidental sudden chemical reactions, which can result in an explosion inside the steam generator, can cause partial deterioration of certain parts of the steam generator, but also, due to the fact of the propagation of the pressure wave in the pipes of the primary circuit caused by the accidental explosion in the steam generator, can cause damage to the core of the reactor or of the intermediate exchangers, or damage to the circulating pumps or any other installation situated in this primary circuit.
It is known to arrange in different parts of the sodium circuit various apparatuses enabling the effects of the sodium water reaction to be limited. It is known, for example, to shunt at the outlet of the sodium of the steam generator a connection to a storage tank closed by a rupture diaphragm, this diaphragm being provided so as to burst rapidly as soon as the pressure of the sodium exceeds a certain value, in order to establish a by-pass for the sodium at excess pressure to a storage tank. It is also known to shunt, on the piping of the primary circuit arriving at the circulation pumps and at the intermediate heat exchangers, an expansion tank enabling the intensity of the pressure wave to be considerably attenuated, before it is propagated into these pumps and these intermediate exchangers.
On the other hand, it is known to arrange the steam generator so that in its upper zone a pocket of inert gas is enclosed, which permits the creation of a free level which very considerably attenuates the pressure waves transmitted in the sodium inlet pipes.
When a primary circuit containing liquid sodium comprises a steam generator the upper part of which contains a gas pocket and comprises on the other hand an expansion tank connected to the circuit through a pipe of large diameter, situated at a certain distance from the steam generator, there has been observed a mass oscillation phenomenon between the two pockets of gas on accidental sodium-water reactions or on simulations of these reactions which are done to test the installation. This swinging phenomenon is similar to that which would exist in a system of communicating tanks closed at their upper part and having a very high gas flow suddenly injected into one of the two tanks. In the same way, in a conventional installation comprising a steam generator provided with a free level and an expansion tank connected in the circuit, the more the inertia of the liquid sodium mass which is situated between the two free levels, the greater the amplitudes of the oscillations. Although the expansion tank is intended to limit the effects of the pressure waves, it is seen that it cannot eliminate considerable excess pressure in the circuits, troublesome for the whole of the installation and particularly for intermediate exchangers which are frequently installed to isolate a first liquid sodium circuit passing into the core of the reactor from the liquid sodium circuit passing into the steam generators.